Open seams are fine for things like stockings, wallhangings and other things that don't get a lot of hands on use.
On a quilt that is used as a quilt, it really does compromise the seam and I avoid that. The exception is for things like an 8 pointed star, then I "swirl" the seam to avoid that bump where they intersect.
That said, I'm not anal about how my seams are pressed....I try to press to the dark, but locking is more important IF that is possible.
In the end, I find that whether the seams lock or don't, and even if they twist between pieces (are laying one way at one end of the seam but the other way at the other end) none of it is really noticeable once the item is quilted.
So while the purist in me gasps a little at pressing seams open, the only real danger is you'll be slightly more likely to have to do a little seam repair somewhere in the future.